This project is designed to elucidate some of the immunochemical and biochemical mechanisms involved in cell-cell interactions between lymphoid cells. Specific emphasis will be placed in defining the role that different cell surface structures present on thymus-derived (T) cells, bone marrow-derived (B) cells and macrophages, play in the mutual recognition between these cells, which result in cellular cooperation leading to antibody formation. The test systems to be utilized in this project are (1) the formation of cell clusters in an in vitro cell culture system and (2) the formation of "T" cell rosettes" between human lymphoid cells and sheep red blood cells. The effects of modification of cell surface antigens of lymphoid cells by appropriate antisera (anti-T cell, anti-B cell, anti-theta, anti-H2, anti- immunoglobulin) and alterations of cell surface carbohydrates by different enzymes (glycosidases, glycosyl transferases, proteases) on cell cluster and rosette formation will be emphasized although other chemical reagents will also be studied for their effects on these two cell aggregation systems.